In ink jet printers, ink jet print cartridges or pens are reciprocated on a carriage to print swaths on an advancing media sheet. Pens typically include an ink chamber partially filled with ink, with a print head having an array of nozzles for expelling ink droplets in a controlled pattern. Some existing pens are self contained units that are discarded when ink is depleted.
More advanced pens employ permanent or rarely-replaced pens and associated replaceable ink supply reservoirs. Upon disconnecting depleted reservoirs and connecting new reservoirs, air bubbles may be admitted into the pen, particularly if the reservoir is connected to the pen via an elongated tube that may be empty of ink and filled with air. As the pen is increasingly filled with air upon each reservoir replacement, or as air enters by any other means, the pen's ink capacity is reduced, and clogging of smaller passages by air bubbles may occur. In addition, when the volume of an air bubble becomes a substantial percentage of the ink chamber volume, external barometric pressure changes such as occur during air travel may cause the bubble to expand enough to expel ink through the nozzles. Therefore, there is a need to remove excess air from the pen.
Simply providing a vent for air to escape is disadvantageous. To prevent ink from "drooling" from the nozzles when the pen is not in use, the pen is maintained at a slight underpressure, which is slightly less than the atmospheric pressure outside the pen. Pens have been provided with air inlet check valves that admit air to avoid excessive back pressure as ink is displaced, but these prevent excess air from escaping. Further, because of the slight under pressure in the pen, any vent would let more air in, instead of letting air out as desired. In the absence of a means to expel excess air, an otherwise-functional pen may fail, requiring replacement before the end of its intended life.
The present invention overcomes the limitations of the prior art by providing an ink jet print cartridge with a body defining an ink chamber and an air outlet. A movable inertia element is connected to the body, and a compressor element is connected to the inertia element and the air outlet. When the pen is accelerated in a selected direction, such as along the carriage path of a printer during printing, the resulting motion of the inertia element operates the compressor to pump a small amount of air from the chamber. To avoid excessive pumping, which may expel ink unintentionally after the air has been expelled, a buoyant or other ink level detector in the cartridge may prevent air pumping when the ink is above a preselected level.